order of arithemtic expression matter

[vale1410]

following program gives an error message: Fatal error: exception Failure("Error: variable X is unbound in an arithmetical expression when grounding p[X \\+ 1].")

#ground p[1], t[2].                                                                                                                       
p[X+1], p[X] :: #ground q[X]. 

it works correctly if order in the generator is swapped:

#ground p[1], t[2].                                                                                                                       
p[X], p[X+1]  :: #ground q[X].